Add contextual options to toggle alignment/snapping properties

[avvvvve]

Context

4.4 introduces new alignment properties for these elements:

• Hairpins, which have both "Snap to previous" and "Snap to next" toggles
  

• Tempo lines, which just has "Snap to next"
  

Requirement 1

Create two new shortcut-assignable actions in Preferences > Shortcuts:

• Toggle snap to previous
• Toggle snap to next

They don't need a default shortcut. These will toggle the properties for both hairpins and tempo lines (for the latter, only "Toggle snap to next").

Requirement 2

Add context menu actions when hairpins and tempo text are right-clicked to control the snapping toggles in properties. The option(s) should be added after "Edit element" and before "Style", with divider lines above/below.

Hairpins:

Tempo text: