Fix #20724 - Remove "Hide/show naturals" in keysig context menu.

In addition to the score-wide style (which is kept without changes), naturals for individual key sigs are controlled by a previous section break: if the previous measure has a section break, naturals are always hidden; otherwise they follow the score settings.

The KeySig::_showNaturals member var is removed, as well as all the code involving it. This should bring a substantial simplification of the matter, while keeping the possibility to control individual key sig instances.

A partial regression is introduced for 1.x scores, which did not have the concept of section break: they will show naturals originally hidden, until the proper section breaks are not added (which it likely to be done anyway, if the score contains several pieces).
fix #15138: double click uneditable element disables further editing
Fix 25245 - Line hooks not saved.

If begin or end hooks are turned on, but their size and type properties are not changed, the hooks themselves are not saved.

See http://musescore.org/en/node/25245 for details.
SLine does not call adjustReadPost() on layout.
Fix the tab order of text line properties dlg box.

The dialogue box for Text Line Properties has an inconsistent tab order across its widgets.

Fixed to normal tab order flow from left to right and top to bottom.
Merge pull request #830 from MarcSabatella/15138-edit-mode

fix #15138: double click uneditable element disables further editing
make clef non editable, generated or not, it's confusing otherwise
Merge pull request #833 from mgavioli/Fix_taborder_in_text_line_props_dlg_box

Fix the tab order of text line properties dlg box.
Merge pull request #832 from mgavioli/fix_SLine_adjustReadPos

SLine does not call adjustReadPost() on layout.
Fix libmscore/copypaste tests
Fix capella/io tests
Merge pull request #780 from mgavioli/Fix_20724_natural_in_keysig_context_Menu

Fix #20724 - Remove "Hide/show naturals" in keysig context menu
Merge pull request #831 from mgavioli/fix_25245_line_hooks_not_saved

Fix #25245 - Line hooks not saved.