Make paint work for RTL languages

[chrisgarrity]

Expected Behavior

Everything should be laid out correctly when using an RTL language such as Hebrew.

• buttons should have the rounded corners on the 'outside':
  
  
• labeled inputs should have margin mirrored
• undo/redo arrows will need to be mirrored
• button icons need margin reversed:
  
• [ ] editing text with the text tool should put the cursor in the correct place (split out to Handle RTL languages in the text tool #615)

Splitting out from scratchfoundation/scratch-gui#1579

Actual Behavior

Describe what actually happens
Layouts specific to LTR look broken on RTL - e.g. margin on the wrong side.

[paulkaplan]

Also the text tool puts the cursor in the wrong place