Add command to toggle selection of all available targets

[agrojean-ledger]

Add command to toggle selection of all available targets for the currently selected app :

• Show a button to toggle selection when hovering on the "select target" item of the main tree view,
• Disable / enable command depending on number of supported targets of current app (only show command if > 1 targets supported)
• Disable / enable eligible extension tasks when toggling the selection of all targets.
• Execute eligible extension tasks normally or for all selected targets.

Note : I have replaced the .png icons by already included vscode.ThemeIcon.