Bookmarks fixes and enhancements

[smizuno]

After some extensive use of bookmarks to explore scattered areas on a map I found some problems with the Bookmarks tool as well as some missing functionality that is very useful to have. I am offering a patch to fix and improve Bookmarks.

Here are the issues:

1. a bookmark fails (silently) to store if it has a single-quote in the name (or project name). The quote mark is now escaped before inserting the text in the database.
   

2. double-click on a bookmark item did not work (help says you can) due to an incorrect slot name. The automatic signal/slot binding didn't work because of this. This fixes #2774
   

3. the stored precision of the extent was limited to 6 digits. I believe that full floating point precision (17 digits) should be preserved, so the number output precision is set to 17.
   

4. have to delete a bookmark, then create a new one in order to update the extent. I have provided an Update button to do this as well as help text on its use.

In the process of working out the coding for the fixes and improvements, I have:

+ reworked some of the Sqlite calls to simplify them as I found them complicated to understand

+ put the call to restorePosition in QgsBookmarks constructor instead of QgisApp. It makes little sense to have the main program call restore if the saving was done in the destructor.

[smizuno]

patch Bookmarks

[jef]

applied in 9ae32d32b2cbc82eee29e550f6b2a2b33d3043e4.