Preview mode

Adding preview mode (#17273 (closed)).

This works as follows:

1. Added a new Preview option under View menu.

2. This has a submenu, where standard sizes are shown, along with a Custom option. If user chooses Custom, she can enter custom resolution. That will get saved and added as an item in the submenu for future use. We remember last 5 resolutions (or whatever number we choose).
   

3. If the user chooses a resolution too big to fit in the currently available viewport (note, this will be smaller than monitor resolution since ParaView's UI items like dock widgets and toolbars eat up space), then we warn the user as follows: The resolution 1920x1080 is larger than the window size. Using 960x540 instead. Thus, ParaView will pick a best-fit resolution for the available viewport size keeping the aspect ratio same as the user's chosen resolution.

4. On selecting the resolution, ParaView enters "Preview mode".

In Preview mode.

1. The viewport size gets locked to the chosen preview resolution.

2. Viewport includes all views in the current tab (unlike "Lock View Size"). Thus, if there are multiple views in the active tab, then all view together will fit the preview resolution. The views will be laid out in proportion to how they were before entering preview mode. If user cares about only 1 view, then she is expected to maximize that view (by clicking ) before entering preview mode.

3. All window decorations are hidden, i.e. the tab-bar (labeled Layout #1..), or the selection buttons, etc. are hidden. We allow user to drag and resize individual views by separators between them.