Language pair install/detection broken in Linux

Something seems to be off when using the plugin under Linux. Trying to install any language pair results in the following:


It looks like it is trying to run 7-zip (as in Windows) and failing. This happens in my laptop, which runs Arch Linux, but I have tested it in virtual machines with Debian and Linux Mint and in none of the cases I have managed to get it to work. Windows, however, seems to be fine (language pairs get downloaded and installed properly).

Furthermore, I have three language pairs installed in /usr/local/share (Arch is not covered by Apertium packaging) which I expect to be detected by the plugin, but they are not.

These are the debug messages given by the plugin when running OmegaT from the terminal:

```
10947: CONFIG: Set root path to: /home/marc/.local/share/apertium-omegat-native
10947: CONFIG: Set package manager to: yum
10947: CONFIG: Set OS to: rpm
10947: Error: 7-Zip Missing: 7-Zip either failed to install or files in your data folder are not executable.
10947: CONFIG: Modes folder did not exist: /home/marc/.local/share/apertium-omegat-native/nightly/usr/share/apertium/modes
```