1. A method for modeling user activity with regard to a plurality of different software applications on a computing device, the method comprising, with the computing device: generating semantic descriptions for each of a plurality of user interactions involving the plurality of different software applications over time, wherein (i) at least a first one of the semantic descriptions relates to gaze-tracking data comprising one or more real-time sensor inputs indicative of a user's gaze in relation to a display of the computing device during a user interaction involving one of the plurality of different software applications; and (ii) at least a second one of the semantic descriptions relates to application event data generated as a result of another interaction involving another one of the plurality of different software applications; algorithmically drawing an inference about a current user activity with the computing device based on at least (i) the first one of the semantic descriptions relating to the gaze-tracking data obtained during the user interaction involving the one of the plurality of different software applications and (ii) the second one of the semantic descriptions relating to the application event data generated as a result of the other interaction involving the other one of the plurality of different software applications and (iii) a recency or frequency of user attention to a user interface element; developing a model of user activity in relation to the computing device based on associations of the semantic descriptions with the gaze-tracking data over time, initially storing the associations in a short-term memory portion of the model, and moving at least a portion of the associations to a long-term memory portion of the model after a period of time; and making the inference available for use by a computer-executable module of the computing device.