Play nicely with other overrides

[jhaygood86]

Fixes #155

1. Reset overrides ONLY overrides permissions controlled by this plug
2. Saving new overrides merges the new values into the existing files in the file

[danirabbit]

Hey nice work! This is really close, but I am noticing an issue where it looks like it's putting together strings with ; in the middle instead of at the end.

For example:

Use flatpak override to add a custom filesystem permission. See that the contents of the overrides file are:

[Context]
filesystems=/path/to/directory;

Toggle a permission off. (I used network). Notice that the the contents are now:

[Context]
filesystems=;/path/to/directory
shared=network

[jhaygood86]

I see what's going on.

[Context]
filesystems=/path/to/directory;

is interpreted as 2 values, one of which is a zero length string. Easy fix though.

[danirabbit]

Nice fix!