GUI: Show warnings on all no-wallet tabs

[kwsantiago]

Fixes #300.

When no wallet is loaded, the warning bar was placed inside no_wallet_group (a child of walletStack). Switching to the Pairing tab swaps m_global_stack to m_page_pairing, hiding walletStack and the warning along with it.

Move m_label_alerts up one level to be a sibling of m_global_stack so it shows on every no-wallet tab. When a wallet is loaded, OverviewPage already has its own alert label, so the WalletFrame-level label is hidden in that case to avoid duplication.

Tested locally with a fake injected warning:

• No wallet, Overview: shown
• No wallet, Pairing: shown (was hidden before)
• Wallet loaded, Overview: shown via OverviewPage (no duplicate)
• -disablewallet: shown via RPCConsole (untouched)

[chrisguida]

After looking into this briefly, I'm still confused as to the bug here... is the problem that the Pairing tab doesn't display warnings? Because that's the only non-wallet tab besides Overview, and Overview already displays warnings properly?

[kwsantiago]

@chrisguida Overview already shows it (fixed in 81d5172 last July); the actual gap is the Pairing tab, which lives in a parallel stack and so the existing alert label gets hidden when you switch to it. This PR fixes that.

[chrisguida]

@luke-jr says the bug is already fixed, do we still need this PR?

[kwsantiago]

@chrisguida , @luke-jr fixed the Overview tab bug, this PR fixes the Pairing tab bug, per my comment above.

[luke-jr]

The Send/Receive/Transactions tabs also don't show the warning. I don't think it would be consistent to put it on Pairing without putting it everywhere...?

[kwsantiago]

@luke-jr Good point. Updated to drop OverviewPage::labelAlerts so WalletFrame's alert label is the single source above every tab (Overview/Send/Receive/Transactions/Pairing).

[chrisguida]

I've tested the new version of this PR with a hardcoded warning in ClientModel::getStatusBarWarnings(), confirmed that the warning is now displayed prominently on all 5 tabs when a wallet is loaded, and on the 2 non-wallet tabs (Overview and Pairing) when no wallet is loaded (the other 3 wallet tabs are inaccessible when no wallet is loaded). Also confirmed that prior to this change, only the Overview tab displays warnings (implemented in 81d5172), in both wallet mode and no-wallet mode.

Technically this PR is larger in scope than the original issue, which only mentions no-wallet tabs, but there are only 2 no-wallet tabs: Overview and Pairing, and Overview was already working properly. The first version of this PR fixed the Pairing tab, which was the exact scope originally outlined in #300 (and still referred to in this PR's title), but fixing warnings on all 5 tabs (regardless of whether a wallet is loaded) strikes me as the correct solution, and this new version is much cleaner than the old version anyway.

Other than the PR's title and top comment needing an update for this new version:

ACK a7685c4