Fixed type associations and added "Default Application"

[tsujan]

This is a relatively big code change:

1. Scheme protocols are added to the end of associations list.
2. A tab for default apps is added. For now, it supports the default browser and email client.
3. Default associations are done in a DE-specific way, so that they don't affect other DEs.
4. The current changes can be undone correctly and reliably.
5. The deprecated class XdgDesktopFileCache and the inefficient class LXQt::SettingsCache are removed from the code. XdgMimeApps is used instead of the former. The latter had bugs and couldn't do a correct and clean resetting; QTemporaryFile is used instead of it.
6. All memory leaks are fixed. They were found both by rereading the code and by using valgrind.

NOTE 1: The patch relies on the latest git libqtxdg and SHOULD NOT be used without it.

NOTE 2: lxqt-config-file-associations only supports setting of default apps. The support for removing/sorting associations is still missing. Association removal is supported by libqtxdg → XdgMimeApps and sorting can be added to it.

Closes lxqt/lxqt#1513

[luis-pereira]

Read the code and played with the app a little bit. Did not find something not Ok.

The Default Apps get and set check code (the browser section) led me to think, that that code and criteria should be in a lib.
A kind of XdgPreferredApps for applications which a simple mimetype association is not enough.

A app writer could write something like:
XdgDesktopFile *prefApp = XdgPreferredApps::webBrowser();

and could also be used as a base for a new qtxdg-mat module to plug into xdg-settings.

Will implement ASAP.

[tsujan]

So fast!

The Default Apps get and set check code (the browser section) led me to think, that that code and criteria should be in a lib.

I can't think of another use for it but if you believe that it should be in the lib, it's OK with me.

IMHO, LXQt::SettingsCache needs more attention (→ lxqt/liblxqt#262). I doubt that it can be fixed based on QSettings because its problem is inherited from QSettings. Maybe, it can be replaced by QTemporaryFile everywhere, in which case, it might not be needed at all.