Introduce equities and securities model

Changes:

1. equities and securities list aren't completely reloaded at each

database change,

2. displayed columns are selectable,
3. no crash after deleting first equity (corner case),
4. no crash after deleting equity in accounts view and then switching to

investment view,

5. context menu in equities tree updates properly when clicked on

columns other than the name column,

6. Call KMyMoneyApp::slotUpdateActions once instead of four times when

selecting equity.

Differential Revision: https://phabricator.kde.org/D7545
Signed-off-by: Łukasz Wojniłowicz <lukasz.wojnilowicz@gmail.com>