Fix tooltip text to follow HIG guidelines (#1050)

[Denay2468]

Fixes part of #1050
Shortened and cleaned up tooltip text across several views to follow HIG guidelines:
∙ Tooltips should be 5–6 words max
∙ Follow header capitalization rule
∙ No trailing punctuation
Changes:
archive_mount.py
∙ “Mount the repository as a folder in the file system” → “Mount repository as folder”
export_window.py
∙ “Disclose your borg passphrase (No passphrase set)” → “No passphrase set”
import_window.py
∙ “Enter passphrase (already loaded from the export file)” → “Passphrase loaded from export file”
∙ “Enter passphrase (already loaded from your keyring)” → “Passphrase loaded from keyring”
∙ “(Name is not used yet)” → “Name not yet used”
archive_tab.ui
∙ “Check the consistency of the repository” → “Check repository consistency”
∙ “Prune the archives in this repository” → “Prune repository archives”
∙ “Optimize disk space by defragmenting the repository” → “Defragment repository”
∙ “Recalculate selected archive’s size(s)” → “Recalculate archive size”
source_tab.ui
∙ “Recalculate source size and file count” → “Recalculate source size”
∙ “Remove the selected source” → “Remove selected source”
diff_result.ui & extract.ui
∙ “Set display mode of diff view” → “Set display mode”
∙ “Collapse All” → “Collapse all”

Before
Standard View (Default)
Simple UI for quick setup.

After
Advanced View (Toggled)
Full access to SSH and Compression settings.

[Denay2468]

All checks passing except macOS test-unit which was cancelled
after 120 minutes due to a CI timeout , not related to my changes.
Lint, integration tests, and Ubuntu unit tests all pass.
Ready for review!

[Denay2468]

Hi @m3nu @ThomasWaldmann , just flagging that I also have
#2479, #2470, and #2445 open if you get a chance to review.
Happy to make any changes needed. Thanks!