Add the QHexEdit widget

Add the QHexEdit widget as some sort of library. Feel free to improve
the project files - I won't, every build system ever made is just
horrible.
Change the internal data type for cell contents to a binary type

Store the data of a DB cell in a QByteArray, i.e. a binary data type,
instead of putting it in a QString, thus converting it to a UTF8 string.

Rewrite the reading and writing of DB cells to correctly handle binary
data containing 0x00 bytes.

Change the edit dialog to actually do all the data checks etc. on a
currently invisible QHexEdit widget instead of a QTextEdit.

All these changes combined make it possible to actually store binary
data without it being corrupted. You can for example import pictures
now, export them and actually open the exported file. So this is an
improvement.
Use different editors in EditDialog depending on the data type

Only show the old text edit widget when there is actually a text to be
edited. If the data is a image file show the image instead of some
random garbage. If it is some other binary data show the hex editor.
Simplify EditDialog code and improve the new multiple editor feature

Remove quite a bit of old code from the EditDialog.

Fix various smaller issues with the multiple editors added in the last
commit.
Allow user to switch between text and hex editor

Allow the user to change between the text editor and the hex editor.

Allow the user to change between insert and overwrite mode in both
editors.

This makes it possible to enter any binary data into an empty cell.
Remove BLOB ToDo from the readme file

The most important commit in this branch...
Use static linking for QHexEdit

Compile and link the QHexEdit as a static library.
Fix minor UI issues with blob editing

By default, use the overwrite mode in the hex edit widget. This way text
and hex editor start in the same mode, making it less confusing to press
the INS key to change the mode.

When editing the data using the hex editor also update the text editor.
Better image file handling in EditDialog

Put the image editor into a scrollarea so the dialog doesn't screw up
when importing a large image file.

When importing a file add a filter to the file dialog to only show image
files.
EditDialog: Don't update the DB when no changes were made

Even when clicking the OK button don't write to the database when no
changes were made.
Merge pull request #8 from MKleusberg/add-blob-support

Add blob support