implemented ImageListFilter

refactored infterface and implemented functions
bug fix: index is not reset when filtering

if a filter is applied, the index is reset to zero if the current image
is filtered out
added image view class

displays a pixmap
added drag-to-scroll for ImageView
changed ImageView::setImage() behaviour

scroll bars are reset when changing pixmaps
pixmap is only changed when the new image is different from the current
image
removed style sheet loading from main.cpp
bug fix: crash when filtering with no loaded files

added NULL checks
refactor: added ImageListFilter::filterByTokens()

moved some of the functionality from filter() to filterByTokens()
updated default stylesheet

changed visual style
merged shuffle()/unshuffle() into one function
added fit-to-width to ImageView

pixmap is resized to fit the width of ImageView;
gets resized when the ImageView is resized
added search icon
bug fix: memory leak

ImageView pixmaps were not being freed from memory
added double-click to toggle fit-to-window
added animated gif support

currently not resizable
updated filter to return image pointer

also modified comments
refactor: rewrote mainwindow class

Moved previous functionality to ImageListFilter and ImageView;
replaced mainwindow.ui with manual initialization
misc improvements
removed mainwindow.ui