UI tweaks

[ghost]

general: Improve tooltips

• Use header capitalization
• Make tooltips shorter

general: Main Menu tweaks

• Allow opening the Main Menu with F10
• Add tooltip to the Main Menu button (HIG advises that all controls in the headerbar must have a tooltip)

notes: Improve AdwStatusPage

• Use header capitalization when applicable
• Add mnemonic to the button
• Shorten the button label

notes: Use header capitalization in menu items

notes: Improve the placeholder text used in the search entry

• Change the placeholder text to be descriptive and to indicate that notes will be searched
• Switch the tree dots to an ellipsis char

References:

• https://developer.gnome.org/hig/guidelines/writing-style.html
• https://developer.gnome.org/hig/patterns/feedback/tooltips.html
• https://developer.gnome.org/hig/patterns/controls/buttons.html
• https://developer.gnome.org/hig/patterns/controls/text-fields.html
• https://developer.gnome.org/hig/patterns/feedback/placeholders.html

[vixalien]

find the changes I suggested.

but these are not fixed, we need to discuss first!