Bug fixed to spreadsheet

[jackratcliffe]

Fixes issues when copying/pasting empty cells to and from excel and when deleting cells from the spreadsheet.

[jayden626]

@jackratcliffe
-Pasting missing values from excel into ICONIC won't overwrite nonnull values
e.g. if Iconic has a dataset in it 1,2,,4 and we copy 1,,3,4 from Excel, we will get 1,2,3,4 instead of the 1,,3,4 that was pasted

Delete context menu button doesn't delete selected cells
Saw you added this (I think), but it's not working. I don't care too much
EDIT: I was looking at the menu bar, not the right click context menu. Context menu works as expected

P.s. can you sneak this in: Context menu > Help states "About Ionic Workbench". Please fix the spelling error to say "About Iconic Workbench"
Change version number to 0.7.0 and add a link to the repo in the help dialog

[jayden626]

See pull request comments

[jackratcliffe]

fixed the missing values not overwriting issue, added top menu bar functionality and updated the about dialogue