screenshot-manager: Allow to save internal screenshots
@gautham_x convinced me that we should save screenshots so the gallery app he might be looking into can present them nicely to the user so here we go.
We separate the file name handling and decision whether we want to copy to the clip board out of phosh_screenshot_manager_do_screenshot
and rather have callers decide.
In the case of internal screenshots triggered by keyboard shortcut or power button menu we save to disk and copy to clipboard.
If we want to land this for 0.43 a review would be appreciated as string freeze is approaching fast.
Closes: #964 (closed)