Add Steinberg locations to VST search paths

In an effort to improve VST integration with LMMS, I believe we should try to locate and utilize VSTs installed in other standard locations.

According to Steinberg, these are the locations for VSTs on Windows: [1]

```
C:\Program Files\Steinberg\{{Application}}\VstPlugins
C:\Program Files\Steinberg\VstPlugins
C:\Program Files\Common Files\Steinberg\VST2
C:\Program Files (x86)\Common Files\Steinberg\VST2
C:\Program Files (x86)\Steinberg\VSTPlugins
```

**Mac is not immediately relevant, we don't support native or Win32 VSTs on Mac yet**

According to Steinberg, these are the locations for VSTs on Mac: [2]

```
/Applications/Cubase.app/vstplugins/ (please verify)
/Applications/Nuendo.app/vstplugins/ (please verify)
/Library/Audio/Plug-Ins/VST
/Users/{{Username}}/Library/Audio/Plug-Ins/VST
/Library/Audio/Plug-Ins/VST3
```

[1] https://www.steinberg.net/nc/en/support/knowledgebase_new/show_details/kb_show/where-are-the-vst2-plug-ins-stored-on-windows.html

[2] https://www.steinberg.net/nc/en/support/knowledgebase_new/show_details/kb_show/where-are-the-vst-plug-ins-stored-on-mac-os-x.html

Start by locating the VST search-path handling in the LMMS source and compare its existing Windows paths with the Steinberg locations listed here. The work is done when supported Windows locations are searched for VSTs without making Mac support part of the change.

Written by the indexing model from the issue text.