initial version of modified Send To button

[rozyczko]

Description

Let's have people look at this redesign and express their opinions on functionality/look.

This PR is for a small change in what the "Send to" button looks like and behaves. QPushButton has been replaced with QToolButton, with drop down menu.
The original functionality is retained - just clicking the button will send the selected dataset(s) to new tab(s) in fitting.
Clicking on the arrow in the right bottom corner of the Send To button will open a small menu allowing users to replace the data with the current dataset instead. This mimicks the "swap" button and slightly declutters the area.

[wpotrzebowski]

The functionality works fine. I haven't looked at the code yet.

The GUI on Mac can potentially be improved

[wpotrzebowski]

It is functional but the button on Mac still doesn't look very nice

[wpotrzebowski]

Looks better but can probably be improved

[wpotrzebowski]

When switching perspectives the error is thrown:

09:44:22 - ERROR: Traceback (most recent call last):
  File "sas/qtgui/MainWindow/DataExplorer.py", line 1045, in updatePerspectiveCombo
  File "sas/qtgui/MainWindow/DataExplorer.py", line 1026, in sendToMenu
RuntimeError: Failed to disconnect signal triggered().

[wpotrzebowski]

Now the error is:

17:19:12 - ERROR: Traceback (most recent call last):
  File "sas/qtgui/Perspectives/Fitting/FittingWidget.py", line 3208, in complete1D
  File "sas/qtgui/Perspectives/Fitting/FittingWidget.py", line 3332, in calculateResiduals
  File "sas/qtgui/Perspectives/Fitting/FittingUtilities.py", line 438, in calculateChi2
ValueError: operands could not be broadcast together with shapes (192,) (0,)

[rozyczko]

Since @wpotrzebowski's observation is not directly related to this PR, can it be merged?