Improve behavior for selecting a menu entry on launch

Previously, it would fail to search a menu for matching menu entries, meaning the app's second positional argument never worked. This commit fixes this error, and simplifies the interface for this by allowing you to specify entry and/or menu in one argument.

You can also now specify an entry only, which will find it in all menus. This should be particularly useful for plasma-desktop#140.

This can be tested with:

Before	After
❌ kmenuedit System org.kde.dolphin.desktop	✔️ kmenuedit org.kde.dolphin.desktop
❌ kmenuedit Utilities org.kde.kwrite.desktop	✔️ kmenuedit Utilties/org.kde.kwrite.desktop
✔️ kmenuedit Education/Mathematics	✔️ kmenuedit Education/Mathematics

^{❌ denotes non-functional behavior (in this case, selection of a menu entry before this MR)}