Merge lp://staging/~mterry/unity-api/warn-on-xapp into lp://staging/unity-api

Add support for isTouchApp property on LauncherItems and support for approving an application before it starts.

This is part of a larger effort to allow warning users before a legacy app is started [1].

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isTouchApp on LauncherItems may be used by unity8 to render the launcher icons differently (though it isn't used now... waiting on design feedback).

This is a distinct setting from the isTouchApp property on ApplicationInfo since LauncherItems represent a potentially-running app, while ApplicationInfos represent a running app.

I considered making a more generic property. Something like "type" which could be "touch", "legacy", or something new and weird in the future. ("gnome" or "android" or something)

However, I felt that a launcher item backend was not necessarily the best place to condense the various information sources into one single answer (what if the .desktop file somehow indicates touch and android support?).

Plus, I felt that the interface should be as dumb as possible, which helps .desktop file backends simply pass the info they see through.

(And it matches the existing ApplicationInfo property)

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The approveApplicationStart API is used to support the new upstart-app-launch round trip blocking when an application is starting (it waits for approval before actually finishing the application startup).

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[1] https://code.launchpad.net/~mterry/unity8/warn-on-xapp/+merge/277915
    https://code.launchpad.net/~mterry/qtmir/warn-on-xapp/+merge/279172
    https://code.launchpad.net/~mterry/ubuntu-app-launch/warn-on-xapp/+merge/278497