Small changes

[magnus-ISU]

Fixes at least one bug: if Dolphin is configured to open inode/directory and I tell Gwenview to open a bunch of crap but not inode/directorys, setting Gwenview for other things would erase Dolphin's entry for inode/directory.

Adds search functionality and restricting of elements to a particular mimegroup; but made the leftmost view a ListView rather than a TreeView, so that you can tell what is there much easier.

Adds a right panel where you can see which associations (if any) an application already has in mimeapps.list.

Adds a help button for people who don't understand exactly what the application does.

Adds command-line option parsing but only one switch, to make it print much more verbose information to the command line.

Adds many comments to the code and makes the code more consistent. Use clang-format to format code, with a similar configuration to the Linux Kernel.

Only allows you to press Set as default once in a row for a QoL improvement and to give feedback that the button was pressed.

Removes a lot of unneccessary data structures from widget.cpp; now all the previous state is contained in one variable. This lets us remove some complexity in some areas. Unfortunately added a few more data structures which needed to store some other information for some of the new features, though, so it isn't actually simpler as a whole.

Oh, btw, now it is called selectdefaultapplication.cpp and not generic widget.cpp.

That's probably it but I might have forgotten something.

[magnus-ISU]

I'm an idiot who opened a PR from master. I wanted to open from change-everything. Will reopen because it won't let me edit it