Allowing exporting from project renderer

[szeli1]

closes #7452
closes #7627
closes #6666

This Pull request adds a way to export individual sample and midi clips. It does this by muting all except one track and setting the loop markers around the exported clip.

How to test:

1. Open lmms
2. Load an audio file in a sample clip / create a midi clip
3. Right click over the sample / midi clip and click on the exporting option
4. Choose the save location and continue
5. The exporting dialog should show up
6. export (and make sure exporting between the markers are enabled)
7. Only the chosen clip should be exported
8. Create a midi clip and "drag n drop" it onto a sample clip
9. Choose the save location and export, make sure to not change the extension
10. The midi clip should be exported and immediately loaded by the sample clip.

This PR allows #7786 to be finished

[sakertooth]

Somewhat a duplicate of #7119 (not sure why I closed the PR).

[szeli1]

Merging with #7119.