Plugin loading broken in flatpaks with 3.17

[yselkowitz]

Describe the bug
The following commit was supposed to fix plugin loading in Fedora:

c8f6c32

Unfortunately this breaks plugin loading in flatpaks, since this returns the Qt location (which is in the runtime) rather than the application's location. This applies to both Flathub flatpaks built with the latest 3.17 snapshot:

[INFO] plugins dir set to:  "/usr/lib/x86_64-linux-gnu/nomacs-plugins"

As well as with Fedora flatpaks (currently WIP):

[INFO] plugins dir set to:  "/usr/lib64/nomacs-plugins"

However, the previous code was also incorrect wrt Fedora native RPM builds, so a simple revert won't fix this. Rather, a different approach appears to be necessary. Either this needs to be a compile-time variable passed from cmake, or a list of possible directories relative to QCoreApplication::applicationDirPath() should be tried until one is found. Accepting an environment variable (e.g. NOMACS_PLUGIN_PATH) as an override, either as an alternative or supplement to those, could also work.

To Reproduce
Steps to reproduce the behavior:

1. Build 3.17.z snapshot as a flatpak
2. Run the flatpak from the command line
3. Note the "plugins dir set to" line.
4. Plugins->Plugin Manager shows no plugins.

Expected behavior
Plugins are discovered and loaded, both in Fedora native builds as well as in flatpaks.

Desktop (please complete the following information):

• OS: Fedora
• Version: 39