Revert "Extra fluxbox safeguard"

This reverts commit afd6e6d3b2bfef49cfee207c37e3780a000e5540.
Revert "Fix selection for fluxbox"

This reverts commit 940f3f85cf6ab5e564ed3ddccc200921b5d8caa3.
Reset centerview scroll to top when collection is changed. Fixes #8583
Do not reset offset in lighttable when coming back from darkroom or map view
Changed lighttable filemanager view so that zooming does not use the upper left image as anchor, but uses the image under the mouse cursor instead.
Main fix is zooming now allows space to be created in fron of the first image, so that the image under the mouse can remain there. In order to achieve this, I found it necessary to move some code around so that the filemanager view is slightly more MVC like, i.e. the expose method does not handle user input any more. The same procedure should be done for the zoomable view, but that is for another commit...
Zooming is ultra smooth now because images are allowed to go outside the borders during the zoom. After zooming the image snap back into the grid.
fixed a problem that a previous zoom operation would mess up a following zoom operation if the view had snapped back to the grid in between. Still has an issue when there is only one image displayed...
Implemented zoom fix for Bug #9168. Smooth zooming code has been removed, so that images always stay perfectly aligned in the grid, due to simpler implementation and broader support in dev community.
fixed zoomable lighttable zoom bug
Fixed issue with empty space before images in lighttable on first load. Also fixed shortcuts g,shift+g,pgup,pgdown. No performance issues have been addressed.
small fix that disallows zooming into empty space above images.
Make the UI behave as expected when an image rating is changed

 When an image rating is changed and it gets out of the current collection the
 current behaviour was not consistent, depending if the image selected was the
 one being changed or not.
Fixed up/down arrow keys in fullscreen preview mode and made left/right arrow keys work the same way. Also implemented the "center scroll" function, which realigns the images with the grid so there is no empty space in front of the first image.
Fixed two bugs concerning keyboard shortcuts for zoomin slider tool in lighttable, which would prevent any images from being shown (offset went outside valid range).
Replacing/moving the realign images shortcut
Moving the realign to grid functionality from the center scroll shortcut/key accelerator to a new one. This commit does it in the non-legacy way.
Mouse click is already handled in button_pressed. Fixes #9223

 Mouse click for selections in lighttable is already handled in button_pressed
 function, so the code for doing so has to be removed from both expose_zoomable
 and expose_filemanager.
Minor style fixes.
Make 'z' also hide the header
Give focus to the center widget when entering lt

This fixes #9285