Add confirmation dialog to mobile settings page

Reason for the change

Currently when pressing the back button the settings page is popped from the page stack. The settings aren't saved or cancelled which is unexpected behaviour. Now we have a message dialog with the option to Save, Discard, or Cancel.

The only issue is that the color scheme menu currently saves changes when changing color scheme.

Requires frameworks/kirigami!1330 (merged) to access the settings page from the sidebar

Test plan

Test Save

1. Open settings page
2. Toggle "Show background on Now Playing page"
3. Click the back button
4. Click Save

• Before: page popped, settings were not changed. Re-opening the settings page showed the unsaved changes
• Now: setting is correctly saved

Test Discard

1. Open settings page
2. Toggle "Show background on Now Playing page"
3. Click the back button
4. Click Discard

• Before: page popped, settings were not changed. Re-opening the settings page showed the unsaved changes
• Now: setting is discarded, now playing page is same as before

Test Cancel

1. Open settings page
2. Toggle "Show background on Now Playing page"
3. Click the back button
4. Click Discard

• Before: page popped, settings were not changed. Re-opening the settings page showed the unsaved changes
• Now: still on settings page. Nothing has been applied

Test music scanning buttons

1. Open settings page
2. Click "Scan For New Music"

• Before: page popped, settings were not changed. Re-opening the settings page showed the unsaved changes
• Now: still on settings page. Nothing has been applied. Notification appears so the user knows the scan has started

Screenshots or screen recordings