Moving empty destructors out of the .cpp files and into headers
allows them to be devirtualized in certain cases.
(When the compiler can't "see" a function in a header, it must largely
assume it's some black box that the linker will resolve.)
While we're at it, use C++11's `= default` to define empty virtual
desturctors for us.
For some classes (e.g., Piano), nothing is derived from it, so we can
mark the class as final and remove any explicit virtual dtor.
There are many other places where this can be done, but this is a large
enough patch as-is.
Move the search bar on top of the file browser for the following sidebar
windows:
* "My Projects"
* "My Samples"
* "My Presets"
* "My Home"
* "My Computer"
Add the greyed out text "Search" to the search text edit.
The text is only shown as long as no text is entered in the search field.
Also rename some variable names to something more meaningful. Rename the
member m_l of FileBrowser to m_fileBrowserTreeWidget. Rename the
following local variables in the constructor of FileBrowser:
* ops -> searchWidget
* opl -> searchWidgetLayout
