Allow only one HTTP poll at a time per extension

Without this change, long-running poll requests (such as those resulting in a timeout) can cause arbitrarily long request queues, eventually leading to spontaneous termination of the application.
Comment formatting tweak
Add block color editor
Fix layout for show button in small stage mode
Conditionally add or skip the extension add button
Now using setSVGData() to properly clear a costume that failed to load.
Fix renaming sounds in nested blocks
Now checking mic for null before starting recording mode.
Fix for parameter focusing.

Fixes #414
Consolidated code

Still fixes #414
Merge pull request #393 from LLK/block-color-editor

Add block color editor to the Edit menu
Merge pull request #416 from MrSprite/procedureFocus

Fix for parameter focusing
Merge pull request #406 from LLK/properly_set_empty_svg

Use setSVGData() to properly clear a costume that failed to load
Merge pull request #413 from LLK/fix_mic_exception

Now checking mic for null before starting recording mode.
Merge pull request #407 from LLK/fix-renaming-sounds

Fix renaming sounds in nested blocks
Merge pull request #405 from LLK/conditional-add-extension-button

Conditionally add or skip the extension add button
Merge pull request #401 from LLK/sprite-info-small

Fix layout for show button in small stage mode
Merge pull request #336 from LLK/extension-poll-wait

Allow only one HTTP poll at a time per extension
Refactored single file loads to use a utility function.
This didn't need to be here