Some more changes, which are rather opinionated, so best decided about by
maintainer :)

• bump min cmake version to 3.0 (2.* is really old, 3.0 standard across kde)
• use cmake VERSION feature now 3.0 is used
• define min deps of Qt5 & KF5 Qt 5.7 by QT_DISABLE_DEPRECATED_BEFORE KF5 5.18 rather random, no idea what a sane base is or what code relies on
• drop unneeded include(FindGettext)
• set CMAKE_MODULE_PATH completly in single place
• use GenericName entry instead of Comment in desktop filename entry seems used in todays app menus, e.g. openSUSE patches in this entry
• add FileTools as another category (picked from xdg menu-spec)

Please tell which ones you do not like or would like differently, would then
do anything okayed in own commits to master.