MS Edge: support find in browse mode

[michaelDCurran]

Find (NVDA+control+f) etc must be supported in browse mode for Microsoft Edge.

To do this, UIATextInfo::find must be implemented making use of IUIAutomationTextRange::findText.

Once implemented, this will expose a bug in Microsoft Edge where findText matches on hidden text (such as display:none) that is not exposed via the UIAutomation text pattern. This has been reported as MS bug 9888158.
P2 as this feature is pretty standard for browse mode.