Some UI tweaks

ui: "Marking upgradable" message

Marking upgradable may take some time (lot of upgradable packages, conflicts)
and user didn't know why the UI is not responsive at that time.

ui: new help line

Shorten "Preview/Download/Install/Remove Pkgs" label to "Preview/Apply"
Add "Mark Upgradable" shortcut info

ui: new actions menu order

OLD:

Install/remove packages
Update package list
--
Mark Upgradable
Forget new packages
Cancel pending actions
Clean package cache
Clean obsolete files
--
Reload package cache
Play Minesweeper
--
Become root
Quit

NEW:

Update package list
Forget new packages
--
Mark Upgradable
Install/remove packages
Cancel pending actions
--
Clean obsolete files
Clean package cache
Reload package cache
--
Play Minesweeper
--
Become root
Quit

ui: rename menu item "Install/remove packages" to "Preview/apply changes"

Rename menu item "Install/remove packages" to "Preview/apply changes" to better match the new help line.