Minor bugfixes and improvements

[!!!][TASK] Use core SlidingWindowPagination #1122

Note that this  is a breaking change, if you use the NumberedPagination class, which now has been removed.
Please adapt your code to use the TYPO3 core SlidingWindowPagination instead.

[BUGFIX] use correct execute method for update statement #1127 - thanks @mkarulin
[BUGFIX] Fixed ignore empty sheet exclusion in pluginupdater
[BUGFIX] Fix checkbox display in plugin preview renderer #1123 #1124
[TASK] Ignore empty/invalid FlexForm in plugin updater #1128
[TASK] Fix CSS classes for plugin preview warning #1125
[TASK] Harden plugin preview #1126
[DOCS] Fixed typo in RSS feed documentation #1121 - thanks @medarob