[Bug] Prefs box keyboard accessibility

Version: 0.6.1 as packaged in Ubuntu 24.04

To reproduce:

1. Start Mousepad.
2. Open the Preferences dialog (Alt+E ↑ Enter).
3. Attempt to navigate tabs and individual settings using the keyboard.

Observed:

• None of the labels have accelerators/mnemonics.
• None of the controls can receive keyboard focus except for Color Scheme.

Expected:

• As a minimum, pressing Tab should visit every individual setting on the visible page in top-down order, Shift+Tab in bottom-up order, skipping disabled controls.
• The tab bar should either be focusable or its tabs should be switchable with Ctrl+PgUp/Ctrl+PgDn.

Cause: 23720274 adds <property name="can-focus">False</property> in many places where previously there was none.

Background: Keyboard accessibility is essential for users who cannot use a pointing device for reasons such as limited limb mobility or aiming precision issues (i.e. tremors). It is also beneficial for able-bodied users.