It seems like there is some kind of touch action interference coming
from the blocks panel, where if you touch the tabs on a touchscreen
chromebook/chrome responsive mode, it does not select the tab if the
touch radius is overlapping the blocks panel at all. See gifs in PR for
more explanation. Fixes issue #1283, making chromebooks useable!

If an extension is loaded at project-load time, there won't be any
targets. In that case, `handleExtensionAdded` now skips target-specific
steps.

This change also adds a filter for JSON block info to support upcoming
block separators, which have XML but no JSON.

Revert "Changes to hash should happen through project loader hoc. Changes to the current project id and current project data should happen in a reducer since they are part of global state. That way, the load button can affect the global project data as well."

This reverts commit e4fe143b.

Changes to hash should happen through project loader hoc. Changes to the current project id and current project data should happen in a reducer since they are part of global state. That way, the load button can affect the global project data as well.

Remove .sb from file browser filter since we can't support them yet. Remove .zip from the same. Save project as .sb3 (and untitled-project).

Added:
- Opera (including Opera Mini)
- Silk

Not Added:
- Vivialdi - appears to be ‘Chrome’ in `platform.name`

Load button file input should only accept specific file extensions. Update load button to use new revamped loadProject function from VM. Loading a project via load button should clear the url hash (without triggering a reload or the project-loader-hoc).

Removing JSZip dependency. VM will unpack the zip (for now, until this functionality gets moved to scratch-storage).

Add switching to costumes tab to surprise button also. All backdrop adding buttons should switch tabs.

Send flag to vm to indicate that mouse-up event was also the end of a drag. VM should use this flag to selectively activate the sprite click hat.