Improvements/Fixes to sample projects

The Speckle Playground sample project has some rough edges that may put off new users. This issue is a maser issue for small tweaks that could be made to improve the experience.

Selection:

[ ] Selection graphics have a phantom mirrored version.
[ ] Selection graphics slow performance.
[ ] Selection graphics not always clear what is selected.

Extra buttons:

[ ] Open in browser button for sender/receiver.
[ ] Gravity on/off toggle button.
[ ] Zoom extent/reset to origin button? sometimes you can get lost.

Bugs:

[x] It is possible to zoom while hovering over the UI (or outside the Game window).
[x] Receiving branch selection sometimes displays the wrong branch. (I think maybe the order is reversed)
[x] https://github.com/specklesystems/speckle-unity/issues/81

Moving

[ ] Keyboard controls are generally a little clunky.
[ ] Missing Up/Down control on Q,E keys.

New Features:

[ ] Sending branch selection not implemented. (see #81)
[ ] Show more details about a stream/commit (preview, collaborations, etc) this is a sample project after all.
[ ] Allow for commit selection on receiver (we already allow for branch selection).
[ ] Display Speckle Properties when selecting object? maybe difficult?

Other feature suggestions can be discussed under this issue.

specklesystems / speckle-unity

Improvements/Fixes to sample projects #77