support for pde:// browser protocol (LINUX)

[peonix0]

I have added the support of pde:// browser protocol for linux(Fixes: #674 )
which enables the opening of *.pde file from web URLs using the pde:// protocol.

as from previous PR#689 discussion, we still have two undesired behavior-

1. on executing xdg-open pde:///path/to/sketchbook/SimpleExport/SimpleExport.pde it open ups two different window, one with desired file other one with empty file.

2. on visiting xdg-open pde://sableraf.github.io/testURIscheme/sketchBundle.pdez it opens sketchBundle.pdez only if processing is already in run.

[SableRaf]

Thanks @lala-lala-lori! Please note that merging your PR may take a little while as we work through other issues. Thanks for your contribution.

[SableRaf]

cc @evan-goode since you weighed in on the previous version of this contribution, would you mind having a look at this new Pull Request? Thanks!

[evan-goode]

LGTM!
These underlying cause of these issues

on executing xdg-open pde:///path/to/sketchbook/SimpleExport/SimpleExport.pde it open ups two different window, one with desired file other one with empty file.

on visiting xdg-open pde://sableraf.github.io/testURIscheme/sketchBundle.pdez it opens sketchBundle.pdez only if processing is already in run.

seem to be unrelated to the changes made in this PR.

[sampottinger]

Thanks for this contribution @lala-lala-lori !

[benfry]

Awesome, thank you! This is merged for 4.3.